A spin catalytic material for selective CO2 reduction, its preparation method and application

Abstract

The present invention discloses a spin catalytic material for selective CO2 reduction, its preparation method and application. By combining mechanically induced spin transition with a two-dimensional layered framework in the crystalline state, high-selectivity CO2 reduction based on the spin transition of active metal centers is achieved. The molecular formula of the single crystal material is: [Fe(μ-bib)2(SCN)2] n (1), where the μ-bib ligand is 4,4'-bis(1H-imidazole-1-substituted)-1,1'-biphenyl; after ultrasonic exfoliation of the oxidized complex o-1, the metal-organic layer nanocatalytic material o-2 can be obtained, realizing partial self-rotation of the catalytic center from high spin (s = 5 / 2) to low spin (s = 1 / 2), showing excellent catalytic performance for the selective photoreduction of CO2 to CO. The selectivity increases from 50% for the high-spin species to 91.6% for the mixed-spin nanolayer, having good application prospects in the development of new energy and environmental governance.

Description

Technical Field

[0001] The present invention belongs to the field of CO2 reduction catalytic materials, and particularly relates to a selective CO2 reduction spin catalytic material, a preparation method thereof, and an application thereof. Background Art

[0002] Climate change is a global issue faced by humanity. With the increase in carbon dioxide emissions from various countries, greenhouse gases have soared, threatening the life system. Against this backdrop, China announced at the United Nations General Assembly its goal of achieving carbon peak before 2030 and carbon neutrality before 2060. To achieve this goal, an important task is to achieve carbon capture and carbon utilization.

[0003] As a catalyst of transition metal complexes or isolated cluster compounds with an extended framework structure, it has received extensive attention in recent years due to its abundant earth resources, low cost, and energy conservation. However, the photoreduction of CO2 in aqueous solution requires a multi-electron reduction process and competes with the reduction of water. Therefore, the photoreduction products of CO2 in aqueous solution are usually interfered by impurities (usually the product contains a mixture of CO, H2, CH4, HCOOH, etc.). Although people are committed to designing an efficient photocatalyst for reducing carbon dioxide, from the perspective of actual solar energy conversion into fuel, it is challenging to establish a catalytic system that can selectively convert carbon dioxide into specific products.

[0004] It is generally believed that the performance of photocatalysts is closely related to the electronic configuration (such as spin state) of metal sites, especially for three-dimensional transition metal organic framework materials. Traditional regulation strategies include changing the valence state of metal atoms, which has been proven to be used to regulate the electronic structure of the whole system. However, it is still challenging to regulate the metal spin state while keeping the valence state of transition metals unchanged and improving the catalytic activity. Summary of the Invention

[0005] In order to regulate the metal spin state while keeping the valence state of transition metals unchanged and improve the catalytic activity, the present invention provides a preparation method and an application of a spin catalytic material for highly selective CO2 reduction.

[0006] The above technical objectives of the present invention are achieved through the following technical solutions:

[0007] A selective CO2 reduction spin catalytic material, the spin catalytic material is made by oxidizing a single crystal material or peeling it after oxidation, and the molecular formula of the spin catalytic material is: [Fe(μ-bib)2(SCN)2] n (1), where the μ-bib ligand is 4,4'-bis(1H-imidazol-1-yl)-1,1'-biphenyl.

[0008] Preferably, the single crystal material is monoclinic, space group C2 / c, and the unit cell parameters are α = 90°, β = 95.524(3)°, γ = 90°, Z = 4,

[0009] Preferably, the Fe atoms in the spin-catalytic material have a distorted octahedral configuration coordinated with six N atoms, where two N atoms are from SCN - , and four N atoms are from the μ-bib ligand; the μ-bib ligand provides two N atoms from the terminal imidazole groups, and two N atoms connect adjacent Fe atoms to form a two-dimensional square lattice metal-organic layer, and the two-dimensional square lattice metal-organic layer is exfoliated to form an ultrathin metal-organic layer.

[0010] The present invention also provides a preparation method of the spin-catalytic material for selective CO2 reduction, including the following steps:

[0011] (1) Preparation of the organic μ-bib ligand:

[0012] 4,4'-Dibromobiphenyl, imidazole, anhydrous potassium carbonate, and anhydrous copper sulfate are placed in a Schlenk flask. After heating and reacting, it is washed with water, and the residue is extracted with ethanol. The organic layer is evaporated to dryness to obtain a crude product, and then recrystallized with methanol and water to obtain 4,4'-bis(1H-imidazol-1-yl)-1,1'-biphenyl;

[0013] (2) Preparation of the single crystal material:

[0014] Using the diffusion layering method, an aqueous solution containing FeCl2·4H2O, a mixed solution of water and methanol, and a methanol solution containing the μ-bib ligand and KSCN are successively placed in a test tube to form a lower layer, a middle layer, and an upper layer, and a light yellow single crystal material is obtained;

[0015] (3) Preparation of the oxidized complex o-1:

[0016] The single crystal material is exposed to air to obtain the oxidized complex o-1;

[0017] (4) Preparation of the nano-sized catalytic material:

[0018] The complex o-1 is ground and ultrasonically exfoliated to obtain the nano-sized catalytic material o-2.

[0019] Preferably, the structure of the nano-sized catalytic material o-2 is an ultrathin metal-organic nano-layer, the structure of the complex o-1 is a two-dimensional square lattice metal-organic layer with a thickness of 4.2 - 4.4 nm, and the thickness of the complex o-1 is 8 - 12 times that of the nano-sized catalytic material o-2.

[0020] Preferably, in step (1), the heating reaction is carried out under a nitrogen atmosphere, the heating temperature is 170-190 °C, and the molar ratio of 4,4'-dibromobiphenyl, imidazole, anhydrous potassium carbonate and anhydrous copper sulfate is 100:(400-440):(300-320):2.

[0021] Preferably, in step (2), the molar ratio of FeCl2·4H2O, μ-bib ligand, and KSCN is (75-125):(190-200):(190-200).

[0022] Preferably, in step (4), the active site of complex o-1 is 100% high-spin Fe 3+ , the active site of the nanostructured catalytic material o-2 is 45-55% high-spin Fe 3+ and 55-45% low-spin Fe 3+ mixed, and the ability of the nanostructured catalytic material o-2 to selectively catalyze the photoreduction of CO2 is higher than that of complex o-1.

[0023] Preferably, the catalytic performance of complex o-1 and the nanostructured catalytic material o-2 can be maintained for at least 5 consecutive cycles.

[0024] The present invention also provides the application of the spin catalytic material prepared by this preparation method in selective CO2 reduction.

[0025] Compared with the prior art, the advantages of the present invention are as follows:

[0026] The raw materials required for the synthesis of the spin catalytic material prepared in the present invention are cheap, the method is simple, and the yield is high. The present invention proves that it is possible to realize the transformation from completely high-spin (HS, s = 5 / 2) to mixed spin (mixed spin, s = 5 / 2 and 1 / 2) by peeling the bulk crystal complex o-1 into a nanostructured catalytic material o-2 with an ultrathin metal-organic layer structure, and the selectivity is increased from 50% high-spin to 91.6% mixed-spin nanolayer, so as to realize the regulation of the active site Fe in the catalytic material without changing the valence state of the transition metal 3+ spin state, and compared with the corresponding bulk crystal, the result of the spin transition significantly enhances the selectivity of CO2 reduction to CO, and has good application prospects in the development of new energy and environmental governance. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The drawings herein are incorporated into the specification and constitute a part of the specification, showing embodiments in accordance with the present invention, and are used together with the specification to explain the principles of the present invention, wherein:

[0028] Figure 1 is the spin catalytic material [Fe(μ-bib)2(SCN)2] in Example 1 of the present applicationn (1) Single crystal structure diagram;

[0029] Figure 2 Transmission electron microscope image of complex o-1 in Example 1 of this application;

[0030] Figure 3 Atomic force microscope image of complex o-1 in Example 1 of this application;

[0031] Figure 4 Ten-layer structure diagram of complex o-1 in Example 1 of this application;

[0032] Figure 5 Before-and-after comparison diagram of complex o-1 ground and exfoliated into nanosized catalytic material o-2 in Example 1 of this application;

[0033] Figure 6 Before-and-after magnetic susceptibility curve diagram of complex o-1 ground and exfoliated into nanosized catalytic material o-2 in Example 1 of this application;

[0034] Figure 7 Before-and-after Mössbauer spectra of complex o-1 ground and exfoliated into nanosized catalytic material o-2 in Example 1 of this application;

[0035] Figure 8 Spin catalytic material in Example 1 of this application is converted from single crystal material [Fe(μ-bib)2(SCN)2] n (1) into electron structure transformation diagram of complex o-1 and nanosized catalytic material o-2;

[0036] Figure 9 Catalytic process diagram of complex o-1 at different times in Example 1 of this application;

[0037] Figure 10 Catalytic process diagram of nanosized catalytic material o-2 at different times after grinding and exfoliation in Example 1 of this application;

[0038] Figure 11 Gas production diagram of complex o-1 repeating 5 times of photocatalytic reaction in Example 1 of this application;

[0039] Figure 12 Gas production diagram of nanosized catalytic material o-2 repeating 5 times of photocatalytic reaction after grinding and exfoliation in Example 1 of this application;

[0040] Figure 13 Powder XRD diagrams of complex o-1 and nanosized catalytic material o-2 before and after reaction in Example 1 of this application;

[0041] Figure 14TG curves of complex o-1 (upper curve) and nano-catalytic material o-2 after grinding and exfoliation (lower curve) in Example 1 of this application;

[0042] Figure 15 IR spectra of complex o-1 (upper curve) and nano-catalytic material o-2 after grinding and exfoliation (lower curve) in Example 1 of this application;

[0043] Figure 16 UV diffuse reflectance spectra of o-1 and nano-catalytic material o-2 after grinding and exfoliation in Example 1 of this application;

[0044] Figure 17 Bonding mode diagrams of Fe-3d with different spin states in the iron active center and O-2p in CO2 molecules in nano-catalytic material o-2 after grinding and exfoliation in Example 1 of this application. Detailed implementation manners

[0045] The following disclosure provides many different embodiments or examples for implementing different structures of the present invention. To simplify the disclosure of the present invention, components and settings of specific examples are described below. Of course, they are only examples and are not intended to limit the present invention. In addition, the present invention may repeat reference numerals and / or reference letters in different examples. This repetition is for the purpose of simplification and clarity, and does not itself indicate the relationship between various embodiments and / or settings discussed.

[0046] With 3d n (8 > n > 3) transition metal atoms with electron configurations adopt high-spin (HS) or low-spin (LS) configurations at room temperature under a specified ligand field strength. Due to the spin flip of the transferred electrons during the d-orbital electron transfer from the t 2g orbital to the e g orbital, metal complexes exhibit changes in spin states between HS and LS configurations. This phenomenon is called spin crossover (SCO, also known as spin transition). The occurrence of SCO is usually due to external stimuli such as temperature, pressure, light irradiation, etc. For example, Fe 3+ The change in spin state induced by the HS→LS transition causes adjustments in unoccupied orbitals. Since its energy is more matched, this orbital is considered more likely to accept electrons from CO2-related adsorbates than other orbitals. Therefore, the spin transition model provides a potential opportunity to improve catalytic activity at the single-atom level.

[0047] To further elaborate on the technical means and effects adopted by the present invention to achieve the predetermined invention purpose, a selective CO2 reduction spin catalytic material and its preparation method and application according to the present invention are described in detail below in terms of its specific implementation manners, features and effects.

[0048] The material sources in the specific implementation are shown in Table 1:

[0049] Table 1 Material components, purities, and sources of the present invention

[0050] Material components Purity Commercially available source 4,4'-Dibromobiphenyl 98％ Anhui Zesheng Technology Co., Ltd. Imidazole 98％ Anhui Zesheng Technology Co., Ltd. Anhydrous potassium carbonate Analytical pure Sinopharm Chemical Reagent Co., Ltd. Anhydrous copper sulfate 98％ Anhui Zesheng Technology Co., Ltd. Ethanol Analytical pure Sinopharm Chemical Reagent Co., Ltd. Methanol Analytical pure Sinopharm Chemical Reagent Co., Ltd. <![CDATA[FeCl2·4H2O]]> Analytical pure Anhui Zesheng Technology Co., Ltd. KSCN Analytical pure Sinopharm Chemical Reagent Co., Ltd.

[0051] A spin-catalytic material for highly selective CO2 reduction, the molecular formula of the single crystal structure of the spin-catalytic material is: [Fe(μ-bib)2(SCN)2] n (1); where the μ-bib ligand is 4,4'-bis(1H-imidazol-1-yl)-1,1'-biphenyl.

[0052] In the structure of this spin-catalytic material, the Fe atom adopts a distorted octahedral {FeN6} coordination configuration, and the six Fe-N bond lengths at the Fe center are in the range. Among the six coordinated N atoms, two nitrogen atoms are from thiocyanate ions, and four nitrogen atoms are from the μ-bib ligand. The μ-bib ligand provides two nitrogen atoms from the terminal imidazole groups, and these two nitrogen atoms connect adjacent Fe atoms to form a two-dimensional square grid-like metal-organic layered structure. Due to the weak π…π stacking interaction between layers in this two-dimensional layered structure, it can be exfoliated into ultrathin metal-organic layers.

[0053] A preparation method of a spin-catalytic material for selective CO2 reduction includes the following operation steps:

[0054] S1: Preparation of the organic ligand μ-bib: Put 4,4'-dibromobiphenyl, imidazole, anhydrous potassium carbonate, and anhydrous copper sulfate in a Schlenk flask in a molar ratio of 100:(400 - 440):(300 - 320):2. After heating and reacting, wash with a large amount of water multiple times until the washing water is no longer blue, and this part removes imidazole and anhydrous copper sulfate. The residue after washing is extracted with ethanol 3 times, the organic layer is collected and evaporated to dryness to obtain a crude product, and recrystallized with methanol and water to obtain a white product, which is 4,4'-bis(1H-imidazol-1-yl)-1,1'-biphenyl.

[0055] The synthesis method for preparing the organic ligand μ-bib is shown in Equation 1: Among them, anhydrous potassium carbonate provides an alkaline and dry reaction environment and does not participate in the reaction; anhydrous copper sulfate is used as a catalyst to improve the reaction efficiency. In an alkaline and dry environment, 4,4'-dibromobiphenyl reacts with imidazole to form 4,4'-bis(1H-imidazol-1-yl)-1,1'-biphenyl.

[0056]

[0057] S2: Preparation of the catalyst single crystal material [Fe(μ-bib)2(SCN)2]n (1): Using the diffusion layering method, an aqueous solution containing FeCl2·4H2O with a molar ratio of (95 - 100):(190 - 200):(190 - 200), water, and a mixed solution of methanol (1:1), and a methanol solution containing μ-bib ligand and KSCN are respectively placed in test tubes to form the lower layer, middle layer, and upper layer. After three weeks, light yellow crystals suitable for X-ray single crystal diffraction are obtained. The μ-bib ligand and KSCN in the upper layer solution will contact the FeCl2·4H2O in the lower layer solution in the middle layer, causing the Fe 2+ ions to act as the central metal ions, and the N atoms in the μ-bib ligand and KSCN to act as coordination atoms. The two undergo a slow self-assembly reaction in the middle layer, ultimately forming a single crystal material. The setting of the solutions in the upper and lower layers of the diffusion layering method is determined by the density of the solutions. The solution with a higher density should be placed in the lower layer, and the solution with a lower density should be placed in the upper layer. If inverted, the two solutions will directly mix, and the target product cannot be obtained.

[0058] S3: Preparation of the oxidized complex o-1: Expose the crystals obtained in step S2 to air for several days to obtain the oxidized complex o-1.

[0059] S4: Preparation of the nano-sized catalytic material o-2: Grind the complex o-1 obtained in step S3 to make its size uniform. Add the ground complex o-1 to absolute ethanol, sonicate for 6 hours, and let it stand for one day. Collect the solid from the upper layer of the mixed solution by centrifugation, and repeat this operation until no more precipitation occurs in the mixture solution. Dry it at room temperature to obtain the nano-sized catalytic material o-2.

[0060] The preparation method for nano-sizing the complex o-1 material of the present invention optimizes the process, reduces the safety hazards in the preparation process of nano-materials, and improves the purity of the target product, making the nano-sized catalytic material o-2 have higher selectivity for CO2 in the photocatalytic system.

[0061] The spin catalytic material prepared by the present invention has two states, namely the oxidized complex o-1 and the exfoliated nano-sized catalytic material o-2. The molecular formulas of both are the same as that of the single crystal material [Fe(μ-bib)2(SCN)2] n (1); after the complex o-1 is exfoliated into the nano-sized catalytic material o-2, the thickness becomes smaller, the molecular structure remains unchanged, but the electronic structure and the electron spin state of the metal center change, and the ability to selectively catalyze the photoreduction of CO2 is also improved.

[0062] Example 1:

[0063] A preparation method for a selective CO2 reduction spin catalytic material, including the following operating steps:

[0064] S1: Preparation of organic ligand μ-bib: 4,4'-Dibromobiphenyl, imidazole, anhydrous potassium carbonate and anhydrous copper sulfate were placed in a Schlenk flask in a molar ratio of 100:420:315:2. After heating and reacting at 180 °C, it was washed with a large amount of water multiple times until the washing water was no longer blue, removing imidazole and anhydrous copper sulfate in this part. The residue after washing was extracted with ethanol 3 times, the organic layer was collected and evaporated to dryness to obtain the crude product, and recrystallized with methanol and water to obtain a white product, which was 4,4'-bis(1H-imidazol-1-yl)-1,1'-biphenyl.

[0065] S2: Preparation of single crystal material [Fe(μ-bib)2(SCN)2] n :

[0066] An aqueous solution (2.0 mL) containing FeCl2·4H2O (0.004 mmol, 0.79 mg) was placed at the bottom of a test tube. A 1.5 mL mixed solvent of anhydrous methanol and water with a volume ratio of 1:1 was used as an intermediate buffer layer and slowly placed in the middle of the test tube. A methanol solution (2.0 mL) containing ligand μ-bib ligand (0.008 mmol, 2.29 mg) and KSCN (0.008 mmol, 0.78 mg) was placed at the top of the test tube. The test tube was sealed and left standing at room temperature for three weeks to obtain a light yellow block crystal material suitable for X-ray single crystal diffraction testing.

[0067] S3: Preparation of oxidized complex o-1 and nano-catalytic material o-2:

[0068] The crystal material obtained in step S2 was exposed to air for several days to obtain the oxidized complex o-1. Weigh 100 mg of the oxidized complex o-1 and grind it to make the crystals of larger particles relatively uniform in size. The ground complex o-1 was added to 100 mL of anhydrous ethanol, and the mixture solution was sonicated for 6 hours and then left standing for one day. The solid at the lower layer of the mixture solution was collected by centrifugation, and this operation was repeated until the mixture solution no longer precipitated, and it was dried at room temperature to obtain the nano-catalytic material o-2.

[0069] The complex o-1 and nano-catalytic material o-2 prepared in Example 1 of the present invention can be used for selective photocatalytic CO2 reaction. The spin transition of metal active sites and the huge specific surface area in the complex o-1 and nano-catalyst material o-2 are the keys to accelerating the catalytic reaction and improving the selectivity, and the selectivity is as high as 91.6%.

[0070] By scanning electron microscope and transmission electron microscope ( Figure 2 ) observation, it was determined that the particle size of the nano-spin catalytic material was 50 - 100 nm. By atomic force microscope ( Figure 3)It was observed that the thickness of complex o-1 with a two-dimensional square grid-like metal-organic layered structure was approximately 4.2 - 4.4 nm, which was equivalent to the thickness of 10 layers of the ultrathin metal-organic layer nanocatalytic material o-2( Figure 4 ).

[0071] The crystal structure of this complex was determined using a Bruker APEX-II CCD instrument with incident radiation to collect diffraction points. The crystal data were refined using the full-matrix least-squares method with the SHELXL program package, and anisotropic refinement was performed on F 2 to obtain the crystallographic data shown in Table 2:

[0072] Table 2 Crystallographic data for single-crystal materials [Fe(μ-bib)2(SCN)2] n (1) and complex o-1

[0073]

[0074]

[0075] a GOF = [∑[w(F o 2 – F c 2 ) 2 / (N obs – N params )] 1 / 2 , based on the data with I > 2σ(I).

[0076] b R1 = ∑||F o | – |F c || / ∑|F o |; wR2 = [∑[w(F o 2 – F c 2 ) 2 / ∑[w(F o 2 ) 2 1 / 2 .

[0077] Furthermore, combining the results of magnetic susceptibility measurements( Figure 6 ) and Mössbauer spectroscopy measurements( Figure 7 ) indicated that there was a nearly 50% spin transition in the fully oxidized complex o-1 before and after exfoliation, from 100% high-spin Fe 3+ ​The complex o-1 (i.e., bulk in the attached figure) is transformed into a nanosized catalytic material o-2 (i.e., nanolayer in the attached figure) which is a mixture of 50% high-spin Fe 3+ and 50% low-spin Fe 3+ .

[0078] The magnetic susceptibility test shows that at 300 K, the magnetic susceptibility of the oxidized complex o-1 is 4.212 cm 3 K mol -1 , which is close to the calculated value (4.375 cm 3+ ) of the magnetic susceptibility of an isolated high-spin Fe 3 (s = 5 / 2, g = 2.0). The magnetic susceptibility of the exfoliated nanosized catalytic material o-2 catalyst at room temperature is only 2.144 cm -1 K mol 3 , because part of the high-spin Fe -1 in the structure is transformed into low-spin Fe 3+ . 3+

[0079] Zero-field 57 Fe spectroscopic tests were carried out on the sample of the complex o-1 before exfoliation and the exfoliated nanosized catalytic material o-2 at room temperature. For the sample of o-1 before exfoliation, the Mossbauer spectrum shows that the ratio of Fe 2+ -HS and Fe 3+ -HS in the freshly prepared sample is 1:4 (HS, high spin; LS, low spin). In the sample o-1 that has been fully oxidized in air, only Fe 3+ -HS is contained. In the two-dimensional nanosized catalytic material o-2 after grinding and exfoliation, the ratio of Fe 3+ -LS and Fe 3+ -HS is 1:1 (spin transition occurs in general iron centers)( Figure 8 ).

[0080] Furthermore, the photocatalytic reduction properties of CO2 were studied on the unexfoliated high-spin sample (o-1) and the exfoliated mixed-spin nanolayer o-2 ( Figure 9 and Figure 10 ). The results show that the catalytic sites with spin transition activity have a great influence on the selectivity of photocatalytic reduction of CO2. For the unexfoliated high-spin sample (o-1), after 4 hours of visible light irradiation, the amount of H2 produced is 10.2 mmol g -1 , and the amount of CO produced is 10.4 mmol g -1 . While the exfoliated mixed-spin nanosized catalytic material o-2 produces 19.7 mmol g -1, the H2 production was only 1.8 mmol g -1 , and the results showed that both the complex o-1 and the nano-catalytic material o-2 had good reducibility for CO2. However, after ultrasonic exfoliation and spin transition, the nano-catalytic material o-2 had a higher reduction selectivity for CO2 and was more inclined to reduce CO2 to CO.

[0081] In the photocatalytic experiment, the oxidized complex o-1 and the nano-catalytic material o-2 were used as catalysts respectively, the photosensitizer was tris(2,2'-bipyridine)ruthenium bis(hexafluorophosphate), the sacrificial agent was BIH, and the solvent system was a mixed solution of acetonitrile and water. In the present invention, both the oxidized complex o-1 and its nano-catalytic material o-2 had a large number of regular layered structures. The layered structure enhanced their accessibility as catalytic active centers, promoted the transport and migration of reaction substrates and products, reduced the recombination of photo-generated electrons and holes, and improved the separation efficiency of photo-generated electrons and holes. Moreover, the two-dimensional nano-catalytic material o-2 had a huge specific surface area, making its structure have high-density and uniformly dispersed catalytic active sites, improving the photocatalytic activity and high selectivity of the material. In the present invention, by subjecting the complex o-1 to nano-treatment to obtain the spin-flipped layered nano-catalytic material o-2, the high selectivity of the nano-catalytic material o-2 for CO2 in the photocatalytic experiment was achieved.

[0082] Furthermore, the cyclic experiment ( Figure 11 and Figure 12 ) evaluation showed that both the un-exfoliated high-spin sample (o-1) and the exfoliated mixed-spin nano-catalytic material o-2 sample could maintain their original catalytic performance in at least 5 consecutive cycles.

[0083] Furthermore, in the present invention, the peak positions of the X-ray powder diffraction patterns ( Figure 13 ) of the catalyst materials before and after exfoliation were basically the same before and after the catalytic reaction, indicating that the crystalline structure of the catalyst material after the catalytic reaction did not change significantly.

[0084] Furthermore, DFT quantum chemical calculations revealed the root cause of the influence of the spin transition of the Fe site on the catalytic activity and catalytic selectivity. The CO2 molecule was adsorbed onto the Fe catalytic center through the Fe–O bond. The low-spin Fe 3+ was more conducive to adsorbing the CO2 molecule and improving the catalytic performance. At the high-spin Fe catalytic center, the Fe-O bond length was while at the low-spin Fe catalytic center, the Fe-O bond length was This indicated that the low-spin iron center had a stronger interaction with the CO2 molecule. The calculation results showed that the adsorption energy of the low-spin iron catalytic center for CO2 was 0.01 eV, while the adsorption energy of the high-spin iron catalytic center for CO2 was -0.06 eV.

[0085] Example 2

[0086] A preparation method of a spin-catalytic material for selective CO2 reduction, comprising the following operating steps:

[0087] S1: Preparation of the organic ligand μ-bib: 4,4'-Dibromobiphenyl, imidazole, anhydrous potassium carbonate and anhydrous copper sulfate are placed in a Schlenk flask in a molar ratio of 100:440:320:2. After heating and reacting at 190 °C, it is washed with a large amount of water multiple times until the washing water is no longer blue, removing imidazole and anhydrous copper sulfate in this part. The residue after washing is extracted with ethanol 3 times, the organic layer is collected and evaporated to dryness to obtain a crude product, and recrystallized with methanol and water to obtain a white product, which is 4,4'-bis(1H-imidazol-1-yl)-1,1'-biphenyl.

[0088] S2: Preparation of the single crystal material [Fe(μ-bib)2(SCN)2] n :

[0089] An aqueous solution (2.0 mL) containing FeCl2·4H2O (0.005 mmol, 0.99 mg) is placed at the bottom of a test tube. A 1.5 mL mixed solvent of anhydrous methanol and water with a volume ratio of 1:1 is used as an intermediate buffer layer and slowly placed in the middle of the test tube. A methanol solution (2.0 mL) containing the ligand μ-bib (0.008 mmol, 2.29 mg) and KSCN (0.008 mmol, 0.78 mg) is placed at the top of the test tube. The test tube is sealed and left to stand at room temperature for three weeks to obtain a light yellow block crystal material suitable for X-ray single crystal diffraction testing.

[0090] S3: Preparation of the oxidized complex o-1 and the nanosized catalytic material o-2:

[0091] The crystal material obtained in step S2 is exposed to air for several days to obtain the oxidized complex o-1. Weigh 100 mg of the oxidized complex o-1 and grind it to make the crystals with larger particles relatively uniform in size. The ground complex o-1 is added to 100 mL of anhydrous ethanol, the mixture solution is ultrasonicated for 6 hours, and then left to stand for one day. The solid in the lower layer of the mixture solution is collected by centrifugation, and this operation is repeated until the mixture solution no longer precipitates, and it is dried at room temperature to obtain the nanosized catalytic material o-2.

[0092] The complex o-1 and the nanosized catalytic material o-2 prepared in Example 2 of the present invention can be used in the selective photocatalytic CO2 reduction reaction. The spin transition of the metal active sites and the large specific surface area in the complex o-1 and the nanosized catalyst material o-2 are the keys to accelerating the catalytic reaction and improving the selectivity, and the selectivity is as high as 89.8%.

[0093] Example 3

[0094] A preparation method of a spin-catalytic material for selective CO2 reduction, comprising the following operating steps:

[0095] S1: Preparation of the organic ligand μ-bib: 4,4'-Dibromobiphenyl, imidazole, anhydrous potassium carbonate and anhydrous copper sulfate are placed in a Schlenk flask in a molar ratio of 100:400:300:2. After heating and reacting at 170 °C, it is washed with a large amount of water multiple times until the washing water is no longer blue, removing imidazole and anhydrous copper sulfate in this part. The residue after washing is extracted with ethanol 3 times, the organic layer is collected and evaporated to dryness to obtain a crude product, and recrystallized with methanol and water to obtain a white product, which is 4,4'-bis(1H-imidazol-1-yl)-1,1'-biphenyl.

[0096] S2: Preparation of the single-crystal material [Fe(μ-bib)2(SCN)2] n :

[0097] An aqueous solution (2.0 mL) containing FeCl2·4H2O (0.004 mmol, 0.79 mg) is placed at the bottom of a test tube. A 1:1 (volume ratio) mixed solvent of 1.5 mL of anhydrous methanol and water is used as an intermediate buffer layer and slowly placed in the middle of the test tube. A methanol solution (2.0 mL) containing the ligand μ-bib (0.007 mmol, 2.01 mg) and KSCN (0.007 mmol, 0.68 mg) is placed at the top of the test tube. The test tube is sealed and left to stand at room temperature for three weeks to obtain a light yellow block crystal material suitable for X-ray single crystal diffraction testing.

[0098] S3: Preparation of the oxidized complex o-1 and the nanostructured catalytic material o-2:

[0099] The crystal material obtained in step S2 is exposed to air for several days to obtain the oxidized complex o-1. Weigh 100 mg of the oxidized complex o-1 and grind it to make the crystals of larger particles relatively uniform in size. The ground complex o-1 is added to 100 mL of anhydrous ethanol, and the mixture solution is ultrasonicated for 6 hours and then left to stand for one day. The solid in the lower layer of the mixture solution is collected by centrifugation, and this operation is repeated until the mixture solution no longer precipitates, and it is dried at room temperature to obtain the nanostructured catalytic material o-2.

[0100] The complex o-1 and the nanostructured catalytic material o-2 prepared in Example 3 of the present invention can be used in the selective photocatalytic CO2 reduction reaction. The spin transition of the metal active sites and the huge specific surface area in the complex o-1 and the nanostructured catalyst material o-2 are the keys to accelerating the catalytic reaction and improving the selectivity, and the selectivity is as high as 90.8%.

[0101] Other embodiments of the present invention will be readily apparent to those skilled in the art upon consideration of the specification and practice of the invention herein. The present invention is intended to cover any variations, uses, or adaptations of the invention following the general principles of the invention and including known common general knowledge or conventional technical means in the technical field of the present invention. The specification and examples are only to be considered as exemplary, and the true scope and spirit of the present invention are pointed out by the claims.

[0102] It should be understood that the present invention is not limited to the exact structures described above and shown in the drawings, and various modifications and changes can be made without departing from its scope. The scope of the present invention is only limited by the appended claims.

Claims

1. A selective CO2 reduction spin catalytic material, characterized in that, The spin catalytic material is made by oxidizing a single crystal material or peeling it after oxidation. The molecular formula of the spin catalytic material is: [Fe(μ-bib)2(SCN)2] n (1), where the μ-bib ligand is 4,4'-bis(1H-imidazol-1-yl)-1,1'-biphenyl.

2. The selective CO2 reduction spin catalytic material according to claim 1, wherein The single crystal material is monoclinic, space group C2 / c, and the unit cell parameters are α = 90°, β = 95.524(3)°, γ = 90°, Z = 4, 3. The spin catalytic material for selective CO2 reduction according to claim 1, characterized in that, The Fe atoms in the spin catalytic material have a distorted octahedral configuration coordinated with six N atoms, where two N atoms are from SCN - , and four N atoms are from the μ-bib ligand; the μ-bib ligand provides two N atoms from the terminal imidazole groups, and the two N atoms connect adjacent Fe atoms to form a two-dimensional square metal-organic layer, and the two-dimensional square metal-organic layer is exfoliated to form an ultrathin metal-organic layer.

4. The preparation method of the selective CO2 reduction spin catalytic material according to any one of claims 1 to 3, characterized in that, It includes the following steps: (1) Prepare an organic μ-bib ligand: Place 4,4'-dibromobiphenyl, imidazole, anhydrous potassium carbonate, and anhydrous copper sulfate in a Schlenk flask. After heating and reacting, wash with water, extract the residue with ethanol, evaporate the organic layer to obtain a crude product, and recrystallize with methanol and water to obtain the 4,4'-bis(1H-imidazol-1-yl)-1,1'-biphenyl; (2) Prepare a single-crystal material: Using the diffusion layering method, sequentially place an aqueous solution containing FeCl2·4H2O, a mixed solution of water and methanol, and a methanol solution containing the μ-bib ligand and KSCN in a test tube to form a lower layer, a middle layer, and an upper layer, and obtain the light yellow single-crystal material; (3) Prepare the oxidized complex o-1: Expose the single-crystal material to air to obtain the oxidized complex o-1; (4) Prepare a nano-sized catalytic material: Grind and ultrasonically exfoliate the complex o-1 to obtain a nano-sized catalytic material o-2.

5. The preparation method of the selective CO2 reduction spin catalytic material according to claim 4, wherein The structure of the nano-sized catalytic material o-2 is an ultrathin metal-organic nano-layer, the structure of the complex o-1 is a two-dimensional square grid-like metal-organic layer with a thickness of 4.2 - 4.4 nm, and the thickness of the complex o-1 is 8 - 12 times that of the nano-sized catalytic material o-2.

6. The preparation method of the selective CO2 reduction spin catalytic material according to claim 4, characterized in that, In step (1), the heating reaction is carried out under a nitrogen atmosphere, the heating temperature is 170 - 190 °C, and the molar ratio of 4,4'-dibromobiphenyl, imidazole, anhydrous potassium carbonate, and anhydrous copper sulfate is 100:(400 - 440):(300 - 320):

2.

7. The preparation method of the selective CO2 reduction spin catalytic material according to claim 4, characterized in that, In step (2), the molar ratio of FeCl2·4H2O, the μ-bib ligand, and KSCN is (75 - 125):(190 - 200):(190 - 200).

8. The preparation method of the selective CO2 reduction spin catalytic material according to claim 4, characterized in that, In step (4), the active site of the complex o-1 is 100% high-spin Fe 3+ , and the active site of the nanostructured catalytic material o-2 is 45-55% high-spin Fe 3+ and 55-45% low-spin Fe 3+ mixed, and the ability of the nanostructured catalytic material o-2 to selectively catalyze CO2 photoreduction is higher than that of the complex o-1.

9. The preparation method of the selective CO2 reduction spin catalytic material according to claim 4, characterized in that, The catalytic performance of the complex o-1 and the nano-sized catalytic material o-2 can be maintained for at least 5 consecutive cycles.

10. Application of the spin catalytic material prepared by the preparation method according to any one of claims 4 - 9 in selective CO2 reduction.

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